1. Field of the Invention
This invention relates to a radio communication apparatus and a radio communication method.
2. Description of the Prior Art
In radio communication systems for high-quality transmission of broad-band digital signals and analog signals the SHF and higher frequency bands are used in light of the breadth of their dedicated bandwidths and the current deficiency of wavelengths in the microwave band. The radio communication systems come under consideration for use particularly in radio LANs, radio home links, radio video transmission systems, and radio car-to-station (or car-to-car) communications systems utilizing millimeter waves. In radio communication systems that use high radio frequency bands, the transmitter generally first generates a modulated signal and conducts multichannel multiplexing in an intermediate frequency band and then uses a local oscillation source to frequency-convert (up-convert) the resulting signal to the used radio frequency band. On the other hand, the receiver uses a local oscillation source to frequency-convert (down-convert) the received radio frequency band signal to the intermediate frequency band and then conducts channel extraction and signal demodulation.
The local oscillation sources used in both the receiver and transmitter must be in synchronism at this time. If the frequency difference or phase difference should fluctuate with time, the fluctuation will be superimposed on the modulated signal after down-conversion and degrade the quality of the demodulated signal. Both the transmitter and the receiver must therefore be equipped with local oscillation sources exhibiting extremely high frequency stability. Therefore, for realizing a local oscillator with high frequency stability, the practice has generally been to adopt a method utilizing a local oscillator stabilized by feedback control using a PLL (Phase Lock Loop) configuration and a method utilizing a synchronized carrier wave regenerative circuit of PLL configuration during signal demodulation.
However, the conventional radio communication systems have the following three problem points:
1) As the used radio frequency band becomes higher, it becomes impossible to realize a frequency-stable local oscillator and increasing local oscillator production cost makes the overall communication system expensive.
2) As the frequency fluctuation and phase noise arising in the local oscillator increase, it becomes difficult to realize synchronized carrier wave regeneration with a PLL configuration in the receiver circuit. Moreover, the receiver configuration becomes complex.
3) Phase noise contained in the local oscillation signals used in the transmitter and receiver degrades signal quality.
This invention was accomplished in light of the foregoing circumstances. An object of the invention is to provide a radio communication apparatus and a radio communication method that enable reduction of transmitter and receiver production cost, simplification of receiver configuration, and high-quality signal transmission.
For achieving this object, this invention provides a radio communication apparatus that down-converts a transmitted radio signal to an intermediate frequency band on a receiving side, which radio communication apparatus comprises a transmitter equipped with an intermediate frequency band modulator for modulating an input signal and outputting it as a modulated intermediate frequency band signal, a local oscillator for outputting an unmodulated carrier, a multiplier for multiplying the modulated intermediate frequency band signal and the unmodulated carrier and outputting a modulated radio frequency band signal, a combiner for combining the modulated radio frequency band signal from the multiplier and the unmodulated carrier output on a branch line from the local oscillator to produce a combined signal and outputting the combined signal, and a transmitting antenna for radio transmission of the combined signal; and a receiver equipped with a receiving antenna for receiving the combined signal from the transmitting antenna, a squarer for obtaining the modulated intermediate frequency band signal from the combined signal received by he receiving antenna, and an intermediate frequency band demodulator for demodulating the modulated intermediate frequency band signal from the squarer.
The invention further provides a radio communication method that down-converts a transmitted radio signal to an intermediate frequency band on a receiving side, which radio communication method comprises a first step of modulating an input signal in an intermediate frequency band to produce a modulated intermediate frequency band signal, using a local oscillation signal to up-convert the modulated intermediate frequency band signal to a modulated radio frequency band signal, and transmitting the local oscillation signal and the modulated radio frequency band signal simultaneously as a radio signal; and a second step of receiving the radio signal, down-converting the received signal by multiplying a component of the local oscillation signal and a component of the modulated radio frequency band signal contained in the received signal to generate a multiplication component and thereby produce a modulated intermediate frequency band signal, and demodulating the modulated intermediate frequency band signal.
In this radio communication method, the modulated radio frequency band signal and the local oscillation signal can be transmitted as orthtogonally polarized waves in the first step, and the modulated radio frequency band signal can be extracted from one and the local oscillation signal from the other of the received orthogonally polarized waves in the second step.
As explained in the foregoing, this invention is adapted so that on the transmitter side a modulated radio frequency band signal and a local oscillator signal are transmitted simultaneously and on the receiver side the transmitted local oscillator signal is used as the local synchronized oscillation. As a result, the receiver does not require a local oscillator. The receiver can therefore be structurally simplified and lowered in production cost.
Further, the local oscillation signal used for down-conversion in the receiver is the same as that used on the transmission side. The modulated IF band signal after down-conversion is therefore not susceptible to quality degradation by phase noise contained in the local oscillator of the transmitter. This makes high-quality signal transmission possible.
The above and other objects and features of the invention will become apparent from the following description made with reference to the drawings.